Cyprinodontiformes vivíparos e ovovivíparos

Livebearer Cyprinodontiformes

Aquariofilia > Salinidade

Aquaristics > Salinity




















A number of Livebearer Cyprinodontiformes are euryhaline species, meaning that they can tolerate or withstand in a wide range of water salinities.

Some species include populations living simultaneously in fresh, brackish, or salt water habitats, comprising all values of the salinity spectrum.

A quantity of other species can even be found primarily in all sorts of protected and semi-protected marine and estuarine habitats, like coastal marshes, mangrove swamps, estuaries, back waters and in brackish water coastal lagoons.

A good number of the Poeciliidae and Anablepidae members exist in quite saline water habitats and can even be found in open ocean environments, at least for short periods, but most often on the neighbouring coastal areas.

A very reduced number of species can even endure in hyper saline ( brine ) niches, namely in desert lakes or ponds.

Although sometimes found in entirely fresh water environments, Belonesox belizanus and a quantity of Jenynsia, Anableps speceis, and several Micropecilia, Gambusia and Poecilia ones, among other less known in the aquarium hobby, are usually found in coastal lagoons, sandy or muddy bottom estuaries mangrove swamps as well as in all potential brackish waters coastal habitats in the tropics and subtropics.

Relatively few species live primarily in clear fast running watercourses, with rocky or large gravel bottoms and pure crystalline fresh water and, as a rule; these are more commonly found among Goodeids.

According to the salt-tolerant classification of fresh-water fishes by George S. Myers ( 1949 ), many of the Livebearer Cyprinodontiformes can be classified as “ secondary “, meaning “ rather strictly confined to fresh water but relatively salt-tolerant, at least for short periods “ and “ sporadic “ or “ fishes which live and breed indifferently in salt or fresh water “.


For the specific purpose of this website, we can define salinity, as the concentration of dissolved salts in water.
As so many species of Livebearer Cyprinodontiformes prefer or require brackish water habitats to stay alive and well, or in order to be kept in hale and hearty conditions, it is advisable to have some notions about this subject and how to control it in the aquarium hobby.
The measurement of salinity in professional circles is usually expressed in “ parts per thousand “ ( ppt ). In the fish keeping hobby, in contrast, the concentration of dissolved salts in water commonly uses the specific gravity ( sg ) notion.
As matter of fact, objects tend to float higher as major is the water salinity, because in saline concentrations, like on sea, the water is heavier per unit of volume ( denser ).
Thanks to this concept it becomes quite easily and quickly to measure salinity through the use of a hydrometer, particularly with those specifically prepared for the hobby, which are reasonably priced and easily obtained.
Later on, some notions about how to use a hydrometer will be explained in plain words.

Conventional classification system of water bodies based upon salinity on the fish keeping hobby

Fresh water

Brackish water

Saline water

Hyper saline water

less than 0.05 %

from 0.05% up to 3 %

from 3% up to 5 %

more than 5 %

less than 0.5 ppt

from 0.5 up to 30 ppt

from 30 up to 50 ppt

more than 50 ppt

Fresh water is generally found in continents and islands. It results from precipitation ( rain and snow fall ) and it flows naturally to the ocean, if not retained.
On the transition places were fresh water is mixed with marine water, we can locate brackish water. But this kind of water can also be found in continental regions, incredibly far away from estuaries and coastal areas. This is the case of some basins or lakes where the magnitude of precipitation is lower than the evaporation level; consequently, when the water evaporates the salts don’t and when this happens… salinity increases.
Besides the above mentioned continental locations, brackish, but primarily saline waters are characteristically marine waters and are concentrated in every ocean around the planet.
Another term for ocean or marine water is Euhaline Sea.
The widespread salinity of seas is generally comprised between 30 and 35 ppt, depending on the ocean spot we are talking about.
Metahaline seas can contain from 36 up to 40 ppt, relating to the concentration of dissolved salts in water.
On the topmost 35 meters of the Dead Sea, in Middle East, the salinity can ranges between 300 and 400 ppt. It’s unquestionably an extreme paradigm of a Brine Sea.
The technical term for salinity in marine environments ( oceans ) is halinity. This is due to the fact that halides - chloride specifically - are the most abundant anions in the mix of dissolved elements of sea water.

Total Molal Composition of Seawater ( salinity = 35 parts per thousand )



Concentration ( mol / kg )

H2O ( Water )


Cl- ( Chloride )


Na+ ( Sodium )


Mg2+ ( Magnesium )


SO42- ( Sulfate )


Ca2+ ( Calcium )


K+ ( Potassium )


CT ( Total Inorganic Carbon )


Br- ( Bromide )


BT ( Total Boron )


Sr2+ ( Strontium )


F- ( Fluoride )



Figure 1

How to use a standard ( universal ) hydrometer to evaluate salinity

Monitoring the water salinity in your tank when you keep fish in brackish or salt water is a must.
To determine water salinity on a professional way, the hobbits needs to make use of equipments which are generally quite expensive.
Therefore, there is a less pricey and more practical approach to determine the water salt concentration in your aquarium, which is by estimating specific gravity with the use of a hydrometer.
A hydrometer is an instrument used to evaluate the specific gravity ( or relative density ) of water and other liquids.
Usually it compares the ratio of the density of one liquid to the density of plain fresh water, but it also works very well for contrast salt ( denser ) water with fresh water.
A standard hydrometer is usually made of glass, but today in the hobby we can find other less precise models made of plastic that have a different way of working.
The traditional type consists of a cylindrical stem and a bulb, weighted with mercury on the base to make it float upright.
The test water is poured into a tall container ( hydrometer jar ), usually in form of a long glass or transparent plastic cylinder supported by a base.
The hydrometer is gently lowered into the water on the jar until it floats freely ( see procedure n.º 3 downwards ).
The reading uses as reference the point at which the surface of the water matches the stem of the hydrometer. Inside the stem, there is a paper scale, so the specific gravity can be read directly.
Specific gravity can be defined as the ratio of water densities at various temperatures, depending on the concentration of dissolved salts on it.
Since there is a direct relation to water temperature, if your hydrometer doesn’t compensate automatically that factor, it should be regulated to the temperature in your tank.
As a rule hydrometers are predominantly calibrated at 15ºC ( 59°F ), so the reading outcome from a standard type still need to be converted to get the actual or true specific gravity of the water.
On the other hand, changes in temperature do affect the densities of materials in general, and the relative densities readings must be corrected at certain temperatures, using a table.
For this purpose you can check your readings with the table of the measured densities of water at various temperatures as the one on the link down below.
But to work with a standard ( universal ) model there are a few simple instructions :

1. Make sure that the instrument ( hydrometer ) is absolutely clean and especially free from previous salt deposits. Wash it with purified freshwater to remove every deposits. If some vestige will not be easily dissolved with fresh water, try washing it with some acid, such as vinegar. A cautious cleanse after every use, will prevent future clean-up complicatedness.

2. Add enough water into the monitoring container so that the hydrometer can float freely, particularly without touching the sides or bottom of it.


3. Lower your hydrometer smoothly, holding it by the very pinnacle of the stem. Release the instrument into the water while simultaneously forcing it a to tiny spin ( in order to displace any bubbles fixed at the hydrometer ).

4. Ensure yourself that there are no visible air bubbles attach to the instrument. Even tiny bubbles will facilitate floating and give you a falsely reading. Pour slowly, and then stir gently.

5. The part of the hydrometer above the water level must be dry and spotless.
Sinking deeply, while place the hydrometer in water, it will leave water remnants on the exposed part. Such extra weigh will drag down the hydrometer and give a fallaciously low specific gravity reading.

6. Make sure that the hydrometer temperature is just about the same as the water’s one.

7. Take a read from the hydrometer at the plane of the water's surface, not along the edge of water where the liquid touches the glass, as the liquid may curve to meet the sides and rises up along the stem of the hydrometer or along the wall of the container ( see red line in figure 1 ).

8. Never leave a hydrometer floating around between uses.

Deposits may form this way and that will be difficult to remove later.

A cautious clean after every use, will prevent a tough concentrated effort after.

If necessary to add some saline solution on your tank’s water, use another container to prepare the blend. Never perform salt mixes straightforward in the aquarium water; unless there are no fish, invertebrates or plants present or in case of emergency. In any circumstances act always very carefully and be gentle.
Using the above mentioned additional container, add salt slowly until the specific gravity reading reaches half of the required limit. Stir the water for a few minutes to facilitate salt assimilation.
Remove and clean the hydrometer, and let the salt water latent for 36 hours.
After this period take another reading.
Continue to add salt until your hydrometer read reaches at the appropriate level for the fish requirements at the environment that will receive the new water. Stir the water for a few minutes to facilitate salt assimilation.
Remove and clean the hydrometer, and let the salt water latent for 24 hours.
After this period take another reading and if the level is proper uses the new water without restraint, otherwise makes the final saline calibration before transfer the water in to the destiny.

Handheld Refractometer

Another option for the concentration of dissolved salts in water in the aquarium hobby is the handheld refractometer. It’s no doubt more expensive than a standard hydrometer, but is becoming more and more popular and soon it will be more low-priced for sure.
The handheld refractometer measures the concentrations of substances in aqueous solutions using refracted light.
The process is a lot easier and more precise than with the standard hydrometer.
You basically place the prism into your tank and watch as an angle is formed on a scale, which indicates precisely the amount of dissolved solids.
After every use just wash the prism and you are ready to take your next reading.
Most basic kits include prism assembly, daylight plate, rubber grip, eyepiece with focus adjustment, and handy case.
The handheld refractometer performs automatic temperature compensation, within a 20 degree range.

A few last words about density measurement

The International System of Units ( SI ) it is the world's most widely used system of measurement.
The SI unit for density is kilograms per cubic meter ( kg/m³ ).
At 3.8° C ( 39.16º F ), water has a density of around 1000 kg/m³, making this a convenient unit, but kilograms per cubic decimetre ( kg/dm³ ), grams per millilitre ( g/mL ), or grams per cubic centimetre ( g/cc or g/cm³ ) are all numerically equivalent ( 1 kg/L = 1 kg/dm³ = 1 g/cm³ = 1 g/mL ).
Determining the specific gravity ( the same as relative density of a material relative to water ) requires little effort and is a simple, as the density only needs to be divided by 1 or 1000, depending on the unit.


Temp ( ºC )

Temp ( ºF )


Correction relative to 15ºC ( 59º F )





























































To compensate or correct your readings regarding temperatres, please follow the Conversion of Hydrometric Readings at any Temperature to Density Table.

To compare your density reading with salinity in ppt, please follow the Corresponding Densities and Salinities Conversion Table.
Mathematically, the formula for density calculation is - (d = m/v); where D is the density, M is the mass and V is the volume.
Although we use the above mentioned metric system, there are other metric units outside the International System of Units.
In U.S. customary units or Imperial Units, the units of density include :

ounces per cubic inch ( oz/cu in )
pounds per cubic inch ( lb/cu in )
pounds per cubic foot ( lb/cu ft )
pounds per cubic yard ( lb/cu yd )
pounds per gallon ( for U.S. or imperial gallons ) ( lb/gal )
pounds per U.S. bushel ( lb/bu ).


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